I left Christianity not because I didn't believe in God, but because I couldn't believe in the Devil. I could never buy into the idea of eternal punishment for temporal action, and I could never buy into the idea of an external cause to temptation or suffering. It seems a way of shirking responsibility.
On the other hand, I could never buy into the idea of heaven or reincarnation as a means of escape. Just as eternal punishment for an individual seems unjust, any sort of eternal reward for an individual rubs me the wrong way.
This isn't necessarily what I believe exists, it's more like how I think things ought to work.
People are reborn but there's no getting out, you never progress, you never leave this world, you just keep coming back, not on your own anyway. Your reward for making the world a better place is to return to a world that is better than it was your last time through. If you made it worse, or even just let things decay for lack of involvement, then your punishment is to come back to a world that's fallen a little further down the hole. So maybe you were an utter shit in your last life and you are reborn as one of the blessed and priviledge - were you punished? No, you don't get punished in this system. What you get is increased odds of coming back miserable. Or happy, if you manage to take your priviledge and blessings and try to repair the collective damage.
Russian roulette - you pull the trigger every time, so are you adding bullets to the Cosmic Gun or taking them away? This life, you put it to your head and hear a click. Empty. But what about the next pull of the trigger? You keep putting ammo in that thing, one of these lives you are going to be born into bullets. If one day the gun comes up totally empty, then maybe that's the day we all go to heaven together. If one day the gun is fully loaded, maybe that's the end of the apocalypse. Until then, I just try to take the bullets out.